22 XII TOPOLOGY AND TOPOLOGICAL ALGEBRA
Let CQxe i be a family of mappings of E into F (resp. R) whose supports
form a locally finite family. Then the sum ]T /a(x) is defined for all x e E,
ae I
because it contains at most finitely many non-zero terms. We denote by
£ fa the function x\-+ £.£(#). If F is R or a normed space (or more generally
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a topological vector space (12.13)), and if each fa is continuous on E, then so is
/= £ y^: for given x e E there exists a neighborhood V of x which meets only
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finitely many of the Supp(^), and hence there is a finite subset H of I such
that/0>)=:£/a(7) for all jeV.
aeH
A continuous partition of unity on E is by definition a family (/^)a 61 of
continuous mappings of E into [0, 1], such that the supports of the fa form a
locally finite family, and such that £ fa(x) = 1 for all jc e E. If (Aa)ae t is an
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open covering of E indexed by the same set I, then the partition of unity
C/a)« e i is said to be subordinate to the covering (Aa)a 6 j if we have Supp(^) c Aa
for all a e I.
(12.6.3) Let (An) Z>£ an at most denumerable locally finite open covering of a
metrizable space E. Then there exists a continuous partition of unity (fn) on E
subordinate to (An).
Let (Brt) be an open covering of E, such that Bw cz Aw for all n (12.6.2). It is
clear that the covering (Bn) is locally finite. By (4.5.2), for each n there exists a
continuous mapping hn:E-» [0, 1] such that hn is equal to 1 on Bn and equal
to 0 on E - Art. If we put gn = (hn — %)+, then Supp(#n) is contained in the
set of points x such that hn(x) ^ i, and hence is contained in An. Let g = £ gn.
n
Since the sets Brt cover E, we have g(x) > 0 for all x e E, and therefore the
functions^ = gjg are defined and continuous on E, and form a partition of
unity with the required properties.
(12.6.4) Let E be a metrizable space, K a compact subset ofE an
a finite covering ofK by open sets ofE. Then there exist m continuous map-
pings fk: E-*> [0, 1] such that Supp(j^) c: Ak for 1 <^ k g m, and such that
m m
%fk(x)^lforallxeEand £/*(*)= IforallxeK.
Take a continuous partition of unity C4)ogfc^m subordinate to the open
covering of E consisting of A0 = E — K and the Ak (1 ^ k g m).e Bk for some k ^ n. Hence the sets BM cover E.